If you are confused about when to use SharePoint Designer and Visual Studio then here are some guidelines which may help you choosing the right technology at the right time. The short answer is

Web Designer=SharePoint Designer whereas Web Developer=Visual Studio

First of all please note that these are two completely different tools to solve two completely different types of problems.

SharePoint Designer is focused on Presentation and Branding. Although, you can some what program a logic to solve some issues (like designing sequential workflows) but it is not good for programming and it is designed to do code free work.

SharePoint Designer is an excellent tool for rebranding the site, customizing the layout, updating CSS and designing Master Pages.

SharePoint Designer can talk to SharePoint natively and can display the pages with full rendering (at least 90%) of web parts.

SharePoint Designer is best suitable for web designers.

Visual Studio 2005/2008 is the tool of choice when you want to extend SharePoint functionality by writing your own custom code.

If you want to design portable workflows, then user Visual Studio.

If you want to create event handlers, custom features, site definitions, custom lists or web parts then use Visual Studio.